Few things
are more annoying than an engine that 'hunts' and
basically runs poor at idle and light throttle. A vacuum
leak is air that enters the engine unmetered. Cracked or
broken vacuum lines, leaking intake manifold or carburetor
gaskets, open carburetor fittings, and loose or missing carburetor
screws are just a few of the causes of vacuum leaks. These
leaks are often difficult to detect, especially those bothering
intake manifold gaskets.

Perhaps the easiest way to determine if an engine has a vacuum
leak is to cup your hand over the choke housing while the engine
is idling. This artificial choke will create a richer
mixture. If the idle speed increases with the richer mixture,
there is a vacuum leak lurking about. Snap-On, Mac, and
other tool emporiums sell inexpensive automotive stethoscopes to
locate vacuum leaks. If these tools are unavailable, you can
substitute a length of vacuum hose.

Place one end of the
hose by your ear and use the other end to search for the leak
while the engine is running. Be careful not to come too close to
the fan or fan belts. If the engine is not a polished show
piece, mix cleaning solvent with automatic transmission
fluid in a squirt oil can. Shoots this mixture
around the intake gasket. If there's a leak, engine speed should
more than likely increase, but the most obvious sign will be
white smoke from the ATF coming from the tailpipes. Other
tricks include shooting aerosol carb cleaner around the suspect
gaskets while the engine is running. When the carb cleaner
hits the leak, engine rpm usually increases, thus pinpointing the
source. The hardest part of fixing an unmetered air leak is
finding it. These few tips should help.

Using Vacuum Gauges:

A good tool to have
(especially on a carbureted engine) is a vacuum gauge. A
vacuum gauge can tell you a lot about what's going on in your
engine. I have an Auto Meter vacuum gauge along with other
gauges in my dash. The vacuum gauge readings are measured in
inches of mercury at sea level. In all 4-cylinder engines, a
slight fluctuation will always be noticeable. This can be
overcome by pinching the gauge hose slightly to enable a steadier
reading.

Below you will find
(8) animated vacuum gauges that will demonstrate various
readings/indications. You may have to refresh the page to
get them all to load.

NORMAL MOTOR:
Steady reading of 17-21 when motor idling.

NORMAL
MOTOR: When throttle is opened and closed rapidly
needle falls to 2 and swings back to 24 or 25, falling back
to normal idle reading Indicates rings and valves ok

POOR
RINGS: Motor idling, hand reading steadily, but
2-4 points lower than normal. This may also indicate poor or
contaminated engine oil, late ignition timing, a leaky
vacuum hose and a leaking gasket between the intake and carb
or throttle body.

POOR
RINGS/POOR OIL: When throttle is opened and closed
rapidly needle falls to 0 and rises to only 24 or less.

STICKING
VALVE: Needle drops occasionally about 4 points at
idle speed. May also be caused by ignition
misfire. Read the spark plugs.

BURNT
VALVE: Needle drops regularly by several points at
idle speed.

LEAKY
VALVE: Needle drops 2-4 points when valve should
close. short circuiting individual spark plugs will indicate
cylinder in which the valve is defective, when engine
idling.

WORN
VALVE GUIDES: Rapid fluctuation of needle between
17 and 21, when engine idling. Exhaust smoke may also
be present. If engine speed goes up with fluctuation,
check for a leaking intake manifold gasket, head gasket,
weak valve springs, burned valves, or ignition misfire.